Multiphase flow meters are used to measure the percentages of gas and liquid flowing out of petroleum wells. Current multiphase flow meters lose accuracy in the measurement of the liquid fraction when the gas fraction becomes predominant. This is illustrated by the trumpet-shaped curve shown in FIG. 1 that represents the typical relative error rate of the measured values of liquid fraction flow as a function of the gas volume fraction (GVF). The curve shows that for a gas volume fraction (GVF) greater than 85%, the relative error rate becomes greater than about 5% and becomes unpredictable for a gas volume fraction (GVF) greater than 99%.
One known technique to solve this problem is to separate a large quantity of gas from the multiphase flow upstream of the flow meter and therefore lower the ratio of gas in the multiphase flow to lower than 85%. Then it becomes possible to measure the percentage amounts of each of the phases with higher accuracy. The actual amount of liquid in the multiphase flow can be computed by determining the amount of gas that was separated from the flow prior to the phase measurements. Unfortunately, this technique requires the use of bulky equipment mounted upstream of the multiphase meter to separate the gas from liquid phase. Such a technique negates the principal advantage associated with such multiphase flow meters, which is the ability to measure the percentage amounts of gas and liquid in a multiphase fluid without the need for separating the gas from the liquid.
A variation of this known technique is to make a partial separation of the gas from a main line of oil, water and gas using a gas diversion line. Such a technique is disclosed in PCT/GB00/01660. Some of the gas is diverted from the main line upstream of the multiphase meter but it is important to mention that there is still some gas flowing through the main line of the multiphase meter.
This partial separation requires the gas to be substantially perfectly dry to avoid losing some of the liquid through the gas diversion line. Consequently, the issue of gas quality (entrainment of liquid inside the gas) becomes as problematic as with a normal separator and the same problems of carry over or carry under are present. Carry over is having some heavier phase going into the gas line (i.e. heavy oil bubbles going into the gas line). Carry under is the opposite, having some gas bubbles going into the oil or water line. Both of these are due to a poor separation of the gas and liquid phases with one predominant phase flowing fast or due to a lack of density contrast between the flowing phases. To cope with this issue, some manufacturers include additional sensors to control or measure the dryness of the extracted gas. Unfortunately, the inclusion of an additional sensor to measure or control dryness increases the complexity of the multiphase flow meter and increases costs.
Another known technique to solve the error rate problem in multiphase flow measurements is to provide upstream of the multiphase flow meter a retention section for loading the multiphase flow meter with some liquid and releasing it in a controlled manner along with the gas in a certain proportion to be able to reduce the relative gas content. Such a solution is illustrated in PCT/GB90/00701 and PCT/US2006/005825 assigned to Framo Engineering and US Published Patent Application No. 2008/0000306 assigned to AGAR Corporation. As indicated previously, such solutions require costly and bulky equipment.
In addition to increased costs, such solutions lack flexibility since the upstream equipment must be specifically designed for an expected flow rate, which of course cannot be guaranteed for the entire life of a well, which may be over 20-50 years.
The multiphase flow meter market is segmented into three main types of multiphase flow meters, including a wet gas multiphase flow meters known as Type I flow meters for measuring only the gas phase flow of the multiphase flow, multiphase flow meters known as type II flow meters capable of measuring liquid and gas, and multiphase flow meters known as type III flow meters capable of measuring and distinguishing all three phases that are usually flowing out of petroleum wells, i.e., oil or condensate, water, and gas.
Unfortunately, all three types of flow meters suffer the same accuracy problem when a high Gas Volume Fraction (GVF) is present.